1. Field of the Invention
The invention relates to the field of ultrasonic scanners and, in particular, to ultrasonic scanners for producing sector scans in an object to be scanned.
2. Prior Art
Dynamic cross-sectional echography (DCE) is a commonly used technique for producing sequential two-dimensional images of cross-sectional slices of the human anatomy by means of ultrasound radiation at a frame rate sufficiently high to enable dynamic visualization of moving organs. Apparatus utilizing DCE are generally called DCE scanners and transmit and receive short, ultrasonic pulses in the form of narrow beams or lines. The reflected signal strength as a function of time, which is converted to a position using a nominal sound speed, is displayed on a cathode ray tube, or other suitable device, in a manner somewhat analogous to radar or sonar displays. While DCE can be used to produce images of any object, it is frequently used for visualization of the heart and main heart vessels.
Existing DCE scanners can be classified according to the geometry of their field of view (linear or sector scanning), according to the means used for scanning that field of view (mechanical or electronic scanning), and according to whether the transducer scans the patient or object through an intervening water bath or by direct contact with the surface of the object as, for example, the skin of a patient using an appropriate contact gel or oil. Linear scanners produce a scan of the anatomy consisting of a set of nominally parallel scan lines, displaced with respect to one another by a line spacing roughly comparable to the effective width of each line, as determined primarily by the transducers used in the apparatus. The cross-section imaged by such scanners is therefore approximately rectangular in shape, its width being determined by the line spacing and total number of lines, while its depth is determined by the lesser of the useful penetration depth of the ultrasound radiation into the body and the unambigous range of the device. Linear scanners are generally used where there is a relatively extended region on the body surface from which access to the parts of interest of the anatomy is possible, as in the abdominal organs. Sector scanners produce a scan of the anatomy consisting of a fan of diverging lines spaced angularly from one another but intersecting (nominally) at a point. The angular spacing being even or uneven depending upon the apparatus and roughly comparable to the effective angular width of each line. The cross-section imaged by such scanners is therefore approximately wedge or pie-shaped, i.e., it is approximately a circular sector, the total angular width of the sector, or sector scan angle, being determined by the angular line spacing and total number of lines, and the sector radius being determined by the lesser of the useful penetration depth of the ultrasound radiation into the body and the unabiguous range of the device. Sector scans are generally used where the anatomical window or region on the body surface from which access to the anatomical part of interest is relatively small, as in the adult heart, the brain and the eye in particular.
A large amount of work has gone into the development of DCE sector scanners. Existing direct contact sector scanners include both phased array and mechanical scanners. In phased array scanners such as those exemplified in articles by M. G. Maginness et al, "State-of-the-art in Two-dimensional Ultrasonic Tranducer Array Technology", Medical Physics, Vol. 3, No. 5, Sept./Oct. 1976, Von Ramm et al, "Cardio-Vascular Diagnosis in the Real Time Ultrasound Imaging", Acoustical Holography, Vol. 6, 1975, and J. Kisslo et al, "Dynamic Cardiac Imaging Using a Phased-Array Transducer System", published by Duke University, Durham, N.C., a large (16-60 element) linear array of small transducers is used, with a variable time (phase) delay inserted between elements of the array both in the transmission and reception of the ultrasound signal, resulting in a transmitted beam and a receiving beam or sensitivity pattern whose direction is determined by the magnitude of the inter-element time delay. In sector scanning using phased array scanners, such scanning is achieved without any mechanical motion of the transducer array which remains in stationary contact with, for example, the patient's skin. Such phased array scanners have, however, several severe practical limitations. One such limitation resides in the relative complexity of the multi-element transducer array and especially of the transmit/receive electronics necessary to achieve electronic beam steering, resulting in a relatively high cost of phased array scanners. In addition, the ultrasonic beam quality in phased array scanners, in terms of lateral resolution and side lobe levels and the possible occurance of grating lobes, is poor compared to that of single transducer scanners, particularly for beam direction angles greater than 30 degrees away from the normal to the transducer, limiting its useful scanning angles to about 60 degrees even though the beam might be steered beyond that limit. Another significant limitation of existing phased array scanners and all direct contact scanners is that the scanned section is centered at the center of the transducer face, essentially at the skin or surface of the object and therefore outside of the patient or object, so that in certain applications close-in structures are not well resolved, while in other applications anatomical structures can limit the field of view of the scanner. This is particularly the case in adult cardiac scanning, where the ultrasonic access window to the heart is generally in the second to fifth intercostal spaces, just to the left of the sternum. In that case, the ribs will tend to limit the scanner field of view, particularly in obese adult patients where the ribs are close to the patient's skin, so that the transducer window cannot readily be pressed into the intercostal space. It would be necessary, in order to avoid the rib interference problem, to have the center of the sector scan replaced somewhat inside the patient, in or near the space between the interfering ribs. Limitations of the scanning sector angle to values significantly below 90 degrees due to rib interference or beam steering limitations or both, can prevent, in many cases, visualization of the entire long dimension of the heart and can seriously affect the diagnostic value of DCE in cardiac examinations as well as in other examinations.
A further limitation of present phased array scanners is that they can only be dynamically focused in range in one lateral dimension, namely in the plane of scanning. Two-dimensional focusing would require a two-dimensional matrix or array of phased transducer elements and is beyond the present commercial state of the art.
Another class of sector scanners are mechanical in nature and can be divided into two classes, oscillating transducer scanners and rotating transducer scanners. An oscillating transducer scanner is exemplified by the scanner described by J. Griffith et al, "A Sector Scanner for Real Time Two-Dimensional Echocardiography", Circulation, Volume XLIX, June 1974, in which a single transducer is oscillated about an axis nominally lying in the front plane and passing through the center of the transducer with an appropriate angle sensor being used to monitor the angular position of the transducer at any time. Contact with the patient is maintained by the use of a gel, and in operation the patient's tissues must conform to the movement of the transducer which is essentially rigid. While the oscillating transducer scanner described by Griffith is of the direct contact variety, oscillating transducer scanners can also be of the non direct or water bath variety as described by A. Ashberg, "Ultrasonic Cinematography of the Living Heart", Ultrasonics, April, 1967, in which the internal structures of the human heart have been investigated by using the ultrasound pulse-echo method and an ultrasound optical mirror system immersed in a water tank having one wall consisting of a thin rubber membrane pressed against the chest wall of the patient through which ultrasonic energy can easily penetrate. These mechanical sector scanners also suffer from a number of limitations and drawbacks which limit their use. Both of the above described mechanical sector scanners suffer the same rib interference limitation as the phased array scanners. In addition, the direct contact mechanical sector scanners are limited in their useful scanning angle by the problems of the moving contact and physical angulation of the transducer away from the skin, in most cases to values of 30 to 45 degrees. A limitation common to both of the mechanical sector scanners described is that their angular rate of sweep is not uniform, since the transducer or mirror system must reverse direction at the end of each sweep in each direction, so that the line density is greatest at the edges of the sector, where it is usually least desirable, and is lowest at the center of the sector, i.e., the center of the region of interest. Concomitant with this limitation is the fact that the alternate direction of sweep means that an area at the end of a sweep is interrogated twice in a very short interval, as the scan crosses it in opposite directions, and is not interrogated again until nearly the duration of two frames. In addition, only the mid-point of the scan is interrogated at a constant frame rate. Another disadvantage of the direct contact of the oscillating transducer scanner described by Griffith arises from the physical transducer motion itself and includes patient discomfort, vibration of the transducer in the operator's hand, and mechanical wear of the transducers moving parts which are subjected to significant forces.
A further limitation of direct contact scanners, including phased array scanners, arises from near field non-uniformities in the so-called Fresnel zone of the transducer or array. As is well known, the acoustic pressure field for an unfocused transducer exhibits large scale oscillations, including a series of peaks and nulls, within a distance D=.sup.r2 /.lambda. from the face of the transducer, where r is the effective radius of the transducer, or array, and .lambda. is the wavelength. Since the region within a distance D/2, which is referred to herein as the "near" Fresnel zone is characterized by particularly large fluctuations in amplitude both laterally and in range, target positions and strengths will be falsely displayed as a sector scan is carried out in that region. For typical transducer radius to wavelength ratio of 10, and typical wavelengths of 0.7mm, the length D/2 of the near Fresnel region, extends 3.5 centimeters in front of the transducer, and this will frequently include portions of the body which are of diagnostic interest.
Another type of mechanical sector scanner is the rotational scanner described by Barber et al, "Duplex Scanner II: For Simultaneous Imaging of Artery Tissues and Flow", IEEE, 1974 Ultrasonics Symposium Proceedings, and by Daigle et al, "A Duplex Scanning System for Pediatric Cardiology", Proceedings 1st Meeting of World Federation for Ultrasound in Medicine and Biology, 1976, which uses a set of transducers mounted on a rotor coupled to the patient through a water column which is separated from the skin surface by a thin silastic or rubber membrane. While the rotating transducer water bath scanner described by Barber, called the "Duplex Echo-Doppler Scanner" permits a stationary contact with the patient and provides a uniform beam spacing or line density as well as uniform sampling, it is severely limited in its application to adult cardiac scanning by the fact that the center or axis of the sector scan is removed or offset from the skin surface by a distance equal to the sum of the rotating scanner radius and the length of the water column, resulting in a severe rib interference problem. The device of Barber et al, is primarily intended for use in pediatric cardiology where rib interference is not serious.
A further limitation of all present mechanical scanners is that they cannot provide a simultaneous M-mode or Doppler scan of any selected line of the scanned sector at rates adequate for measurements of heart valve and heart wall motions. While any line of the sector scan of a mechanical scanner can be sampled at the frame rate of the sector scan itself, typically 20 to 45 frames per second and displayed on an M-mode type display, this rate is too slow since a minimum of 300 frames per second is necessary in order to resolve rapid motions, such as the motion of the mitral valve of the heart, with existing M-mode single beam echocardiographic probes operating at frame rates in excess of 1000 frames per second. In addition, even if such rates could be attained by a mechanical scanner, the unambiguous range, or useful penetration, corresponding to a frame rate of 300 or more frames per second of the 80 to 100 lines typically forming a sector scan would be less than two centimeters, and therefore totally useless. One attempt to provide a Doppler scan in a mechanical scanner is shown in the rotational scanner described by Barber in which an auxiliary transducer operated in the pulsed Doppler mode is provided which permits obtaining information about velocities of blood flow and movement of cardiac structures essentially simultaneously (within less than a millisecond) with the echoamplitude information. However, the Doppler scan in the device described by Barber is not centered around the same point as the echo scan, since the transducer is mounted off to the side of the echo scanning head. Thus, the point of entry of the Doppler beam and the corresponding interrogated volume are different than the point of entry of the echo sounding beam and its corresponding interrogated volume, creating problems both of access and of interpretation as the same line as one of the sector lines is not simultaneously sampled.
The image producing capabilities of current ultrasonic scanners are further limited by the existence of "echo" artifacts which degrade the quality of and complicate the interpretation of the reflected signals from the object being visualized. Such echo artifacts are caused by ultrasound energy being received by a detector which energy is not directly reflected from the body or target under examination. In a system utilizing mirrors and membranes, such as described by Asberg, a portion of the echo artifacts are caused by partial reflection of acoustic pulses along the path of the desired or "target" echoes by the membranes and the mirrors before they reach the body or target under examination. Another portion of the echo artifacts are caused by partial reflection of acoustic pulses from the membranes and mirrors not along the return path of the target echoes but along other paths resulting in reflections from numerous internal surfaces which eventually inpinge upon the detector. A further portion of the echo artifacts results from stray acoustic radiation that is not intercepted by the membranes or mirrors but merely reflects around the scanner with some of it reaching the detector and producing false echoes.
Accordingly, it is a general object of the present invention to provide an improved ultrasonic sector scanner.
It is another object of the present invention to provide a sector which has a sector scan center of focus which can be located in front of the scanner so as to minimize interference problems.
It is a further object of the present invention to provide a sector scanner having a large effective sector scan angle.
It is another object of the present invention to provide a sector scanner which has a stationary contact with the object being scanned and is free of vibration problems.
It is still another object of the present invention to provide a sector scanner which has uniform line density and sampling rate at all angles, a high frame rate, and high quality radiating and receiving beam patterns.
It is a further object of the present invention to provide a sector scanner which can provide a simultaneous M-mode or pulsed Doppler scan of any selected line of the sector scan at a high frame rate comparable to conventional M-mode frame rates or pulsed Doppler systems.
It is another object of the present invention to provide a sector scanner which is free of echo artifacts.
It is a further object of the present invention to provide a sector scanner in which no part of the body of diagnostic interest lies in the near Fresnel zone of large variations of acoustic intensity.